VAST defines a schema as a named record type. Since all types can have a name, we define a schema as a type a that (1) is a record, and (2) has a name. From a data perspective, the schema specifies the structure of a batch of records.
Ideally, each data source defines its own semantically rich schema to retain most of the domain-specific information of the data. This is desirable because accurately modeled data is more productive to work with because it's less error-prone to misinterpret and requires fewer context switches to infer missing gaps. VAST's type system is well-suited for deep domain modeling: it can express structure with lists and records, add meta data to any types via tags, and also support aliasing for building libraries of composable types.
In practice, many tools often "dumb down" their rich internal representation into a generic piece of JSON, CSV, or text. This puts the burden of gaining actionable insights onto the user downstream: either they work with a minimal layer of typing, or they put in effort to (re)apply a coat of typing by writing a schema.
However, writing and managing schemas can quickly escalate: they evolve continuously and induce required changes in downstream analytics. VAST aims to minimize the needed effort to maintain schemas by tracking their lineage, and by making data sources infer a basic schema that serves as reasonable starting point. For example, the JSON reader attempts to parse strings as timestamps, IP address, or subnets, to gather a deeper semantic meaning than "just a string." The idea is to make it easy to get started but still allow for later refinements. You would provide a schema when you would like to boost the semantics of your data, e.g., to imbue meaning into generic string values by creating an alias type, or to enrich types with free-form attributes.
Many data sources emit more than one event in the form of a record, and often contain nested records shared across multiple event types. For example, the majority of Zeek logs have the connection record in common. Factoring this shared record into its own type, and then reusing across all other occurrences makes it easy to perform cross-event connection analysis later on.
You can write schemas manually by providing a module. To find out existing schemas, consult the documentation on introspection.